1. Field of the Invention
The present invention relates to an electronic view finder apparatus that is suitably used in, for example, a video camera apparatus. More particularly, the invention concerns an electronic view finder apparatus adapted to observe a video display apparatus, such as an LCD, having the characteristic of Viewing-Angle by magnifying it with the use of an ocular lens unit, which can enhance the contrast of the image depending upon the characteristic of Viewing-Angle by making proper the relative disposition between the video display apparatus and the ocular lens unit.
2. Description of the Prior Art
Conventionally, in an electronic view finder for use in a video camera apparatus, a small-sized CRT (cathode ray tube), LCD, etc. is mainly used as the video display apparatus; and, it is arranged so that the image of the video display apparatus is observed by being magnified through the use of an ocular lens unit. However, because the CRT has a long size in the depth direction, as measured from the image screen surface, it is unsuitable for being used in a small-sized video camera apparatus. Also, the LCD is small in size and light in weight, and, in addition, the number of pixels thereof has in recent years been remarkably enhanced. Therefore, it also becomes sufficiently practical to perform focusing with respect to the image of the finder, so that the LCD has been widely diffused over a high grade of apparatus as well.
By the way, in the video display apparatus using a CRT, even when the image screen surface is looked at from an oblique direction with the direction in which to observe it being more or less shifted from the front, brightness or contrast of the image does not change sensitively. However, in case of an LCD, when looking at the image screen surface from an oblique direction, the brightness or contrast sensitively changes. In addition, when having observed the image screen surface from the front, the best contrast is not always obtained. There is also a case where the contrast becomes the best when observing the image screen surface from a direction that is oblique by a prescribed degree of angle with respect to a prescribed azimuth angle. This characteristic in which the brightness or contrast depends upon the direction in which to observe is referred to as xe2x80x9cthe characteristic of Viewing-Anglexe2x80x9d.
While there are some kinds of liquid crystals for use in an LCD, the liquid crystal that is most widely used is TN (Twisted Nematic) type liquid crystal. The present invention also concerns an electronic view finder apparatus that uses therein an LCD that uses TN type liquid crystal.
Here, an explanation will be given of the operational principle that the characteristic of Viewing-Angle of the TN type liquid crystal is not symmetrical and, when this liquid crystal is looked at from a prescribed direction and at a prescribed angle, it has the best contrast.
FIG. 4 illustrates the arrangement of the liquid crystal molecules when no voltage is applied to the TN type liquid crystal.
Between two sheets of polarizing plates 10, 11, whose polarization directions intersect each other at a right angle, liquid crystal cells 12 and 13 are clamped in a way so as to be sandwiched therebetween. Regarding the liquid crystal cells 12 and 13, the liquid crystal molecules 14 thereof on one liquid crystal cell 12 side are parallel with the polarizing plate 10 on the light incidence side. On the other hand, on the other liquid crystal cell 13 side, they are twisted through an angle of 90xc2x0 and, in this state, they become parallel with the polarizing plate 11 on the light emission side. Accordingly, the back light that has entered the liquid crystal from the polarizing plate 10 side on the light incidence side is linearly polarized by the polarizing plate 10. Thereby, as in the case of the liquid crystal molecules 14, the back light is twisted (rotated) by 90xc2x0 and goes out of the polarizing plate 11. Namely, the LCD looks white.
FIG. 5 illustrates the arrangement of the liquid crystal molecules when a voltage has been applied across transparent electrodes (not illustrated) between the liquid crystal cells 12 and 13.
According to this arrangement, the liquid crystal molecules 14 become vertical with respect to the liquid crystal cells 12 and 13 along the electric field that has been applied as above. As a result of this, the rotatory action does not occur. Accordingly, the back light that has entered from the polarizing plate 10 side on the light incidence side is interrupted by the polarizing plate 11 on the light emission side. Resultantly, the LCD looks black.
By the way, speaking strictly, the liquid crystal molecules 14 illustrated in FIG. 5 have only their liquid crystal molecules alone that are in contact with the liquid crystal cells 12 and 13 influenced by the inter-molecular attractive force between themselves and high-molecular thin film surfaces coated on the surfaces of the liquid crystal cells. For this reason, those liquid crystal molecules, owing to this influence, don""t completely become vertical with respect to the liquid crystal cells 12 and 13 but become slightly rotated. Namely, the LCD doesn""t completely look black.
FIG. 6 illustrates the relationship between the voltage (V) applied across the liquid crystal cells and the transmittance (T). This graphic diagram is an example of the V-T characteristic (xcex8=0xc2x0) as measured in the vertical direction of the LCD. As apparent from this graphic diagram, applying a high voltage to the liquid crystal cells results in the transmittance becoming zero. However, for the reason that the liquid crystal molecules become slightly rotated, as stated above, that transmittance doesn""t completely become zero. Also, applying a high voltage is not realistic because of the problems with the withstanding voltage of a device constituting the drive circuit, the unnecessary increase in the voltage consumption, etc. Accordingly, in general, the black signal voltage level of the LCD is set to 4 volts.
Also, the white signal voltage of the LCD is set to 1.5 volts, whereby through applying a signal voltage of 4 to 1.5 volts, colors from black to gray to white are displayed.
Here, the degree of excellence of the contrast of the brightness of the LCD is quantitatively defined in terms of the contrast ratio between the transmittance at the time of applying a white signal voltage and the transmittance at the time of applying a black signal voltage. Namely, because the transmittance corresponding to 4 volts is 2%, the contrast ratio in this case is 100/2=50.
FIG. 7 illustrates the V-T characteristic with regard to the oblique direction in addition to the V-T characteristic (xcex8=0xc2x0) with regard to the vertical direction of the LCD of FIG. 6. Namely, as stated previously, even when a high voltage is applied, the liquid crystal molecules that are in contact with the liquid crystal cells are not completely vertical with respect to the liquid crystal cells. Namely, they are more or less inclined and thereby have a certain low level of rotatability. When the LCD is looked at from that inclined direction, that low level of rotatability is relatively mitigated. As a result, even when applying the same black signal voltage, the LCD looks black.
For example, the V-T characteristic (xcex8=10xc2x0) is such that when the black signal voltage is 4 bolts, the transmittance is approximately 0.1%. Therefore, the contrast ratio becomes 100/0.1=1000. The contrast ratio, therefore, is greatly improved. Conversely, when the LCD is looked at from a side of promoting the rotatory polarization, the V-T characteristic (xcex8=xe2x88x9210xc2x0) is such that when the black signal voltage is 4 volts, the transmittance is approximately 10%. Therefore, the contrast ratio becomes 100/10=10, which indicates that the contrast ratio becomes greatly deteriorated.
The present invention has been made in order to solve the above-described problems and has an object to provide an electronic view finder apparatus that by making proper the relative position between the LCD and the ocular lens can mitigate the rotatory polarization and can thereby improve the contrast of the image.
To attain the above object, the present invention provides an electronic view finder apparatus arranged such that the liquid crystal image screen surface has the characteristic of Viewing-Angle so that this image screen surface is seen with its contrast or brightness being different according to the direction in which to see the image screen surface; the center of an opening portion on the light emission side of the ocular lens unit is disposed in a way that the center is shifted from an optical axis of the ocular lens unit; an observer""s eye is thereby guided to a position detached from the position corresponding to the optical axis; and when the observer resultantly is made to see the liquid crystal image screen surface from an oblique direction, the image screen surface is seen with a higher contrast than when the observer has seen the image screen surface from along the optical axis by satisfying the following conditions:
0.3 h less than xcex94 less than 1.5 h
where, under the assumption that xcex8 represents the angle defined by a direction in which the image is most clearly seen with respect to the optical axis and f represents the focal length of the ocular lens unit when the diopter is set to have a value of 0, the amount h of shift between the optical axis for seeing the image at the angle of xcex8 and the visual line is obtained from the h=fxc2x7tan xcex8; and the xcex94 represents the actual amount of shift between the optical axis and the visual line.
According to the above-described electronic view finder apparatus, the video display apparatus, in which the contrast becomes low due to the characteristic of Viewing-Angle when being looked at from its front surface, becomes able to be observed with an optimum contrast.